1. Field or the Invention
The present invention relates to the production of printed circuit boards, and more particularly to methods for providing a desired conductive pattern on a substrate board.
2. Description of the Prior Art
The printed circuit board (hereinafter "PCB"), also known as a printed wiring board, has been the foundation of circuit packaging since as early as the 1940s. This well-established method of interconnecting and packaging a variety of electronic components is both space efficient and cost-effective. For those reasons, PCBs are widely used in, for example, computers, consumer electronics, military applications, and a number of other electronics products.
PCBs have many advantages over discrete wire connection of electronic components. For instance, when there is a call for high production of identical electronic components, PCBs have significant advantages. One reason is that, within a single production batch, the electronic components and their arrangement within the PCB are identical for each PCB. Fabrication therefore becomes repetitive and the assembly line may be wholly or partially automated. Also, quality control may be carefully monitored. Such automation and quality control advantages translate to significant cost savings. By contrast, hard wiring is labor intensive and prone to errors during assembly Another reason PCBs are superior to hard wiring is that electronic components on a PCB can be packaged more densely, thereby reducing size and weight. Due to these advantages of PCBs, any improvements in the processing of PCBs are highly desirable.
A conventional PCB employs a laminate structure having one or more layers of conductive copper foil laminated onto a "substrate" board. The substrate board is itself typically a laminate, made from an electrical insulator such as paper, woven glass cloth, or thermosetting or thermoplastic polymer, which insulator is impregnated with resin. The resin-impregnated board, without the copper is commonly called a "prepreg," in the industry. The copper foil is laminated onto the "prepreg" to provide the conductive layer of the PCB which carries the electrical signals between the electronic components that are later mounted on the PCB. The desired circuit network is transferred onto the copper-clad board by etching the copper layer into the desired conductive pattern.
The two basic processes for creating the conductive pattern in the copper foil are screen printing and photoprinting. In either case, a stencil or template duplicating the desired circuit network is prepared ahead of time. In the screen printing process, liquid resist material is forced through the stencil to create an image of the desired circuit network on the copper foil layer. Then, the resist is cured and different chemicals are applied to strip away the copper not covered by the resist material. The balance of the copper that remains after stripping and curing becomes the circuit network. In the photoprinting process, the desired circuit network is photographed onto a board after it has been coated with a light-sensitive photoresist material. After exposure to light, the circuit pattern in the photoresist material hardens and protects the copper thereunder from etching chemicals that are subsequently added. After passage of time, the chemicals dissolve the unexposed portions of the photoresist material along with the copper layer underneath. The remaining pattern formed in the copper layer is the current-carrying circuit. Conventional methods used to make a circuit network include combinations of these printing and etching methods, as well as plating.
In the final step, electronic components like transistors, resistors, diodes, etc. are soldered on to the board. Those electronic components have leads that pass through holes drilled in the board where they are then soldered (called the through-hold design). More recently, the components are commonly soldered directly onto the exposed copper cladding without use of drilled holes. This method is called Surface Mount Technology (SMT).
Although the conventional methods of manufacturing PCBs are quite versatile and highly evolved, there are a number of limitations and disadvantages. One disadvantage of patterning PCBs through chemical etching is that it is expensive due to the chemicals consumed and the sophisticated processes used. Also, it is inherently a multi-step and relatively slow process which significantly reduces PCB throughout. In addition, specially-equipped manufacturing facilities and highly-skilled technicians are required to support certain segments of such an operation. Furthermore, the toxicity of expended chemicals and their byproducts limits the efficacy of reclamation procedures. Indeed, concerns for the environment and the ecosystem make disposal of these toxic chemicals a nontrivial and costly task. Lastly, certain conductors are not well adapted to etching techniques. In particular, high temperature superconductors are difficult to chemically etch while retaining the desired superconductivity. For such superconductors, other patterning techniques must be devised.
Several improved methods of producing a printed circuit board are described in a co-pending application of Peter L. Jurisich, Ser. No. 07/552,774, which application was filed Jul. 16, 1990 and is entitled "Mechanical Method For Printed Circuit Board Patterning". Such co-pending application describes a method of fabricating a circuit network for a printed circuit board through mechanical embossment. A conductive layer is laminated onto an electrically insulating board, and simultaneously an impression of a desired circuit network is embossed into the conductive layer and board, creating high and low regions therein. Portions of the conductive layer situated in the high regions are then ground off, leaving the low regions which form the desired circuit network. In a second method described in the co-pending application, the embossment process is modified to shear the high regions away from the low regions The resulting circuit network is of that portion of the conductive layer situated in the high regions. In a third method described in the co-pending application, adhesive is deposited on the conductive layer in a pattern that is an image of the desired circuit network. The conductive layer is then laminated to the board. Portions of the conductive layer that are unbonded are then ground off, with the balance of the conductive layer being left behind to form the desired circuit network.
The methods described in co-pending application Ser. No. 07/552,774 are advantageous over various prior art techniques in producing printed circuit boards having desired conductive patterns thereon. However, it may be desirable for certain applications to provide alternative methods of fabrication. For example, rather than defining a desired conductive pattern on a conductive layer and then removing the remaining portions of such layer using various techniques, it may be advantageous to provide a surface of a substrate board or other portion of the printed circuit board with pattern defining portions that are then treated to form the conductive pattern. Such techniques may be advantageous from the standpoint of cost, ease of manufacture and the nature of the printed circuit board thereby produced.